1. Field of the Invention
The present invention relates to a motor control device that converts alternating current (AC) power supplied from an AC side into direct current (DC) power to output the DC power, and then, further inverts the DC power into AC power for driving a motor to supply the AC power to the motor. In particular, the present invention relates to the motor control device that includes an electric storage device storing energy for protecting operation at the time of power failure, and a resistance discharge device consuming DC power of a DC link by resistance discharge.
2. Description of the Related Art
In a motor control device that drives a motor in a machine tool, a forging-heading machine, an injection molding machine, an industrial machine, or each of various robots, AC power input from an AC power supply side is converted into DC power once, and is then further inverted into AC power. This AC power is used as drive power for the motor that is provided for each drive axis. The motor control device includes a rectifier and an inverter. The rectifier rectifies the AC power supplied from the AC power supply side at which a three-phase AC input power supply exists, to output DC power. The inverter is connected to a DC link on a DC side of the rectifier. The inverter performs interconversion of electric power between DC power of the DC link and drive power for the motor or regenerative electric power, which are AC power. The motor control device controls a speed or torque of the motor, or a position of a rotor of the motor, the motor being connected to an AC side of the inverter.
Recently, due to demand for saving energy, in many cases, a motor control device is provided with a rectifier by an electric power supply regeneration method that can return, to the AC power supply side, regenerative energy generated at the time of motor deceleration.
FIG. 16 illustrates a configuration of a general motor control device that drives a plurality of motors. The motor control device 101 includes a rectifier 11 and an inverter 12. The rectifier 11 rectifies AC power from a commercial three-phase AC input power supply 3 to output DC power. The inverter 12 is connected to a DC link 13 on a DC side of the rectifier 11. The inverter 12 inverts the DC power output from the rectifier 11 into AC power of a desired voltage and a desired frequency that is supplied as drive power for a motor 2, or converts AC power regenerated from the motor 2 into DC power. The motor control device 101 controls a speed or torque of the motor 2, or a position of a rotor of the motor 2, the motor 2 being connected to an AC side of the inverter 12. The same number of the inverters 12 as the number of the motors 2 are provided for individually supplying drive power to each of the motors 2 respectively provided in correspondence with a plurality of drive axes, to drive and control the motors 2. However, in many cases, one rectifier 11 is provided for a plurality of the inverters 12 in order to reduce cost and an occupied space of the motor control device 101.
An higher-level control device (not illustrated) transmits a motor drive command to each inverter 12 to control operation of inverting DC power into AC power by each inverter 12 (more concretely, switching operation of switching devices in each inverter 12). Thereby, the higher-level control device performs control such that each inverter 12 inverts DC power at the DC link 13 to output desired AC power. The motor 2 operates with drive power which is the AC power output from the inverter 12. Accordingly, controlling AC power output from the inverter 12 can control a speed or torque of the motor 2, or a position of the rotor of the motor 2, the motor 2 being connected to the AC side of the inverter 12. When the motor control device 101 controls the motor to be decelerated, regenerative electric power is generated from the motor 2. This regenerative electric power passes through the inverter 12 to be converted into DC power, returned to the DC link 13, and further inverted into AC power by the rectifier 11. Then, the AC power is returned to an AC power supply side where a three-phase AC input power supply 3 exists.
In such a motor control device 101, when power failure occurs on the AC power supply side of the rectifier 11 and an input power supply voltage declines, normal operation of the motor 2 is difficult to be continued. For this reason, some inconvenience occur. For example, the motor 2, the motor control device 101 driving the motor 2, a tool connected to the motor 2 driven by the motor control device 101, a machining target machined by the tool, a manufacturing line including the motor control device 101, and the like are damaged or deformed. Accordingly, a power failure detecting unit 14 is provided on the AC power supply side of the rectifier 11 to monitor whether or not power failure occurs on the AC power supply side of the rectifier 11. When the power failure detecting unit 14 detects power failure occurrence, the motor control device 101 operates so as to perform protecting operation for avoiding the above-mentioned inconvenience or suppressing the inconvenience to the minimum. As a device that stores energy used for performing the protecting operation at the time of power failure, an electric storage device 17 is connected, via a charging unit 15 and a discharging device 16, to the DC link 13 between the rectifier 11 and the inverter 12. A charging-and-discharging control unit 118 outputs, to the charging unit 15, a charging command that causes the electric storage device 17 to store DC power. The charging-and-discharging control unit 118 outputs, to a discharging unit 16, a discharging start command that causes the DC power stored in the electric storage device 17 to be discharged to the DC link 13.
By the operation of the discharging unit 16, the DC electric power stored in the electric storage device 17 is supplied to the DC link 13. For example, when power failure occurs on the AC power supply side of the rectifier 11, or when the three-phase AC input power supply 3 existing on the AC power supply side is an electric generator, regenerative energy is difficult to be returned to the AC power supply side where the three-phase AC input power supply 3 exists, and there is a possibility that a DC voltage at the DC link 13 rises to a voltage beyond withstanding voltages of the rectifier 11, the switching devices in the inverter 12, and the like. By taking it into account, to deal with such a situation, a resistance discharge device 19 is provided at the DC link 13 between the rectifier 11 and the inverter 12. Thereby, a measure to consume DC power supplied from the electric storage device 17, and regenerative electric power generated at the time of motor deceleration, as heat energy of a resistance (also referred to as “regenerative resistance”) in the resistance discharge device 19.
In the motor control device 101 including the above-described configuration, before the motor 2 is driven, the charging-and-discharging control unit 118 outputs a charging command to the charging unit 15, and DC power at the DC link 13 is stored in the electric storage device 17. After a charged voltage of the electric storage device 17 reaches a desired DC voltage, the motor control device 101 starts to drive and control the motor 2. Since a charged voltage of the electric storage device 17 declines due to natural discharge and the like, the electric storage device 17 is continuously charged by the charging unit 15 also during a period in which the motor 2 is being driven. In other words, when a charged voltage of the electric storage device 17 becomes equal to or lower than a predetermined voltage, the charging-and-discharging control unit 118 outputs a charging command to the charging unit 15 so that the electric storage device 17 is charged.
When the power failure detecting unit 14 detects power failure occurrence, the charging-and-discharging control unit 118 stops output of a charging command to the charging unit 15, and outputs a discharging start command to the discharging unit 16. Thereby, charging of the electric storage device 17 by the charging unit 15 is stopped, and DC power stored in the electric storage device 17 is discharged to the DC link 13 via the discharging unit 16. The higher-level control device (not illustrated) outputs, to each inverter 12, a motor driving command for performing protecting operation for avoiding the inconvenience or suppressing the inconvenience to the minimum, the inconvenience being breakage or the like of the motor 2, the motor control device 101 driving the motor 2, a tool connected to the motor 2 driven by the motor control device 101, a machining target machined by the tool, a manufacturing line including the motor control device 101, and the like. On the basis of the motor driving command for the protecting operation, the inverter 12 performs switching operation of the switching devices in the inverter 12 to invert DC power at the DC link 13 into AC power that just enables the motor 2 to perform the protecting operation, and the inverter 12 outputs the inverted AC power. By the operation of the discharging unit 16, DC power stored in the electric storage device 17 is supplied to the DC link 13. However, it is possible for a DC voltage at the DC link 13 to exceed withstanding voltages of the rectifier 11, the switching devices in the inverter 12, and the like, and to rise to a voltage that allows each device to be broken. To avoid this, at the time of the protecting operation, when a DC voltage at the DC link 13 reaches a specified voltage or more, the resistance discharge device 19 converts DC power at the DC link 13 into heat energy to consume the DC power.
One example of the charging unit 15, the discharging unit 16, and the resistance discharge device 19 will be described as follows.
For example, a motor control device according to Japanese Unexamined Patent Publication No. 2012-158483 includes an electric storage device at a DC link between a rectifier and an inverter, and boosts a voltage at the DC link to store, in the electric storage device, energy for protecting operation at the time of power failure. Thereby, storage energy at a unit volume is maximized to reduce a volume and cost of the electric storage device.
FIG. 17 is a circuit diagram illustrating one concrete example of the charging unit in the motor control device according to Japanese Unexamined Patent Publication No. 2012-158483. In the motor control device according to Japanese Unexamined Patent Publication No. 2012-158483, the charging unit 15 with a boosting function of charging the electric storage device (not illustrated) at a voltage higher than a DC voltage at the DC link (not illustrated) is configured by a buck-boost chopper circuit including switches S1 and S2, diodes D1 and D2, and an inductor L1. When the electric storage device is charged, on the basis of comparison between a charged voltage of the electric storage device and a DC voltage at the DC link, on-off control is performed on the switches S1 and S2. When a charged voltage of the electric storage device is smaller than a DC voltage at the DC link, the switch S2 is kept normally off, and on-off control is performed on the switch S1 at a predetermined duty ratio to charge the electric storage device. Then, when a charged voltage of the electric storage device becomes larger than a DC voltage at the DC link, the switch S1 is kept normally on, and on-off control is performed on the switch S2 at a predetermined duty ratio to charge the electric storage device. By such a charging unit 15, the electric storage device is able to be charged such that a voltage of the electric storage device is boosted to a voltage higher than a DC voltage at the DC link 13. For example, when the electric storage device is a capacitor, energy P[J] stored in this electric storage device is proportional to the square of a charged voltage V as expressed by the Expression 1, the charged voltage of the electric storage device being V[V], a capacitor capacitance being C[F].P=½CV2  (1)
In the invention according to Japanese Unexamined Patent Publication No. 2012-158483, a voltage of the electric storage device is boosted close to a withstanding voltage of each device of the rectifier and the inverter connected to the DC link, thereby, stored energy in a unit volume can be maximized. Accordingly, it is possible to miniaturize the electric storage, and reduce cost.
FIG. 18 is a circuit diagram schematically illustrating the discharging unit according to the invention described in Japanese Unexamined Patent Publication No. 11-178245. According to the invention described in Japanese Unexamined Patent Publication No. 11-178245, the discharging unit 16 is configured by a thyristor S3 and an inductor L2. When power failure occurs at an AC power supply side of a rectifier (not illustrated), the thyristor S3 is turned on to make short-circuiting between a DC link (not illustrated) and an electric storage device (not illustrated) to supply DC power stored in the electric storage device to the DC link.
FIG. 19 is a circuit diagram illustrating one concrete example of the resistance discharge device in the motor control device according to Japanese Unexamined Patent Publication No. 2012-158483. FIG. 20 illustrates one example of fluctuation in a DC voltage at the DC link at the time of power failure operation in the one concrete example of the resistance discharge device in the motor control device according to Japanese Unexamined Patent Publication No. 2012-158483. As illustrated in FIG. 19, the resistance discharge device 19 includes a resistance R1, and a switch S4 that makes opening or closing between this resistance R1 and the DC link (not illustrated). After power failure occurrence at the AC power supply side of the rectifier (not illustrated) is detected, when a DC voltage at the DC link exceeds a resistance discharging operation start level set in advance, the switch S4 is closed. Thereby, regenerative energy from the inverter (not illustrated) to the DC link is consumed at the resistance R1 as heat energy so that a DC voltage at the DC link drops. Then, when a DC voltage at the DC link declines to a resistance discharging operation stop level set in advance, the switch S4 is opened. Thereby, owing to regenerative energy from the inverter to the DC link, a DC voltage at the DC link is shifted to rising. Thus, start and stop of the resistance discharging operation of the resistance discharge device 19 causes a DC voltage at the DC link to repeatedly rise and decline between the resistance discharging operation stop level and the resistance discharging operation start level. Generally, as illustrated in FIG. 20, hysteresis is given between the resistance discharging operation start level and the resistance discharging operation stop level such that switching between start and stop of the resistance discharging operation of the resistance discharge device 19 does not occur too often.
For example, as described in Japanese Unexamined Patent Publication No. 2002-338151, an elevator device is proposed. In the elevator device, an electric storage device is connected, via a charging-and-discharging device, to a smoothing capacitor provided between a rectifying unit and an inverter. At the time of regenerative operation, the electric storage device is charged by dropping voltage of the smoothing capacitor. At the time of power running, and at the time of power failure, a voltage of the electric storage device is boosted and discharged to the smoothing capacitor. Accordingly, it is possible to use regenerative energy effectively, without increasing a capacitance of the electric storage device.
Further, for example, as described in Japanese Unexamined Patent Publication No. 2005-192298, an elevator device is proposed. In the elevator device, a rechargeable battery is connected, via a DC-DC converter, to a smoothing capacitor between a converter and an inverter. The rechargeable battery is charged by an initial charging current that is determined by an estimation value of an inverter consuming electric power at the time of power failure, and a voltage of the rechargeable battery. Only at the time of power failure, electric power is supplied from the rechargeable battery. Thereby, at the time of power failure, the rechargeable battery is in a fully charged state so that long time operation can be performed with certainty.
Furthermore, for example, as described in Japanese Unexamined Patent Publication No. 2009-261161, there is a method in which a capacitor is provided between a converter unit and an inverter unit, and at the time of detecting voltage drop at a DC side of a rectifier caused by decline in an AC voltage at an AC side of the rectifier, energy stored in the capacitor is used to continue operation of a motor.
However, in a case in which an electric storage device is boosted and charged by using a charging unit with a boosting function of charging an electric storage device at a voltage higher than a DC voltage at a DC link, there is a possibility that when DC power is started to be supplied from the electric storage device to the DC link immediately after power failure occurs at the AC power supply side of the rectifier, energy for the protection operation at the time of power failure is consumed by the resistance discharge device, and therefore, intended protecting operation is difficult to be performed.
FIG. 21 illustrates voltage fluctuation in the DC link and voltage fluctuation in the electric storage device when power failure occurs under motor stop, and DC power is supplied from the electric storage device to the DC link, in a situation where the electric storage device is boosted and charged by using the charging unit with the boosting function. When power failure occurs at the AC power supply side of the rectifier under motor stop, and DC power is started to be supplied from the electric storage device to the DC link, a DC voltage at the DC link rises to a voltage close to an initial charged voltage of the electric storage device since capacitor capacitance of the electric storage device is generally larger than the sum of capacitor capacitance of the rectifier and the inverter. At the time of power failure, electric power for driving motor is difficult to be supplied from the AC power supply side, and accordingly, the protecting operation is performed by energy stored in the electric storage device and the DC link.
However, when power failure occurs at the AC power supply side of the rectifier under motor rotation, energy of the electric storage device is difficult to be used effectively. FIG. 22A and FIG. 22B illustrate voltage fluctuation in the DC link and voltage fluctuation in the electric storage device when power failure occurs under motor rotation, and DC power is supplied from the electric storage device to the DC link, in a situation where the electric storage device is boosted and charged by using the charging unit with the boosting function. FIG. 22A illustrates a motor speed, and FIG. 22B illustrates voltages at the DC link and the electric storage device. A resistance discharging operation start level of the resistance discharge device is set at a level that protects each device of the rectifier and the inverter connected to the DC link. Meanwhile, the electric storage device is boosted and charged almost to a withstand voltage of each device of the rectifier and the inverter connected to the DC link, by the charging unit. Accordingly, the resistance discharging operation start level of the resistance discharge device is set at a voltage that is almost equal to or larger than a voltage at which the electric storage device is charged by boosting, as illustrated in FIG. 22B. When power failure occurs under motor rotation, the protecting operation is started, and the motor is decelerated. Further, at the same time as the motor deceleration, DC power is started to be supplied from the electric storage device to the DC link. In addition, regenerative electric power from the motor is also returned to the DC link. For this reason, a DC voltage at the DC link exceeds the initial charged voltage of the electric storage device. Then, when a DC voltage at the DC link reaches the resistance discharging operation start level set in advance, the resistance discharge device starts the resistance discharging operation. Thereby, DC power of the electric storage device and the DC link is consumed as heat energy, and a DC voltage at the DC link is shifted to dropping. When the resistance discharging operation is started once, the resistance discharging operation is performed until voltages at the DC link and the electric storage device decline to the resistance discharging operation stop level. After that, in a period in which the motor is decelerated, a DC voltage at the DC link repeatedly rises and declines between the resistance discharging operation stop level and the resistance discharging operation start level. In the period in which the motor is decelerated, a voltage of the electric storage device does not rise, and is maintained at the resistance discharging operation stop level.
Thus, when DC power is started to be supplied from the electric storage device to the DC link immediately after power failure occurrence, a DC voltage at the DC link rises, and when a DC voltage at the DC link reaches the resistance discharging operation start level, the resistance discharging operation of the resistance discharge device is started so that a DC voltage at the DC link declines. The decline of a DC voltage at the DC link continues until a DC voltage at the DC link reaches the resistance discharging operation stop level. The resistance discharging operation stop level is lower than the initial charged voltage of the electric storage device. Accordingly, energy almost corresponding to a voltage from the initial charged voltage of the electric storage device to the resistance discharging operation stop level is wastefully consumed by the resistance discharge device. In other words, a part of energy of the electric storage device stored for the protecting operation is wastefully consumed as heat energy by the resistance discharge device. For this reason, energy efficiency is low. There is a possibility that energy needed for the protecting operation is insufficient, depending on a consumption situation of DC power discharged from the electric storage device, the consumption being performed by the resistance discharge device. When an energy storage capacitance of the electric storage device is designed to have a margin such that energy needed for the protecting operation does not become insufficient, a volume and cost of the electric storage device is increased.